1. Field of the Invention
The present invention relates to a piezoelectric actuator, and a liquid transport apparatus provided with the piezoelectric actuator.
2. Description of the Related Art
Piezoelectric actuators have been hitherto known, which drive various objectives by utilizing the deformation of the piezoelectric layer (piezoelectric strain) brought about when the electric field is allowed to act. For example, Japanese Patent Application Laid-open No. 2006-304588 discloses a piezoelectric actuator for an ink-jet head, wherein the piezoelectric actuator is arranged on a surface of a flow passage unit including nozzles and pressure chambers, for applying the jetting pressure to the ink contained in the pressure chambers formed in the flow passage unit. The piezoelectric actuator is an actuator having the so-called unimorph structure. The piezoelectric actuator has a vibration plate which is made of metal and which is joined to one surface of the flow passage unit to cover the plurality of pressure chambers therewith, a piezoelectric layer which is stacked on the vibration plate, and individual electrodes which are formed in areas on a surface of the piezoelectric layer, the areas being opposed to central portions of the pressure chambers.
In this arrangement, the piezoelectric layer is polarized in the thickness direction. When a predetermined driving voltage is applied between the individual electrode and the vibration plate made of metal (common electrode), the electric field, which is parallel to the thickness direction as the direction of polarization, is allowed to act in the piezoelectric layer interposed by the individual electrode and the vibration plate. Accordingly, the piezoelectric layer is elongated or expanded in the thickness direction as the direction of polarization, and the piezoelectric layer is shrunk in the plane direction or the surface direction perpendicular to the direction of polarization. In this situation, the vibration plate is deformed so that the vibration plate is convex toward the side of the pressure chamber in the area facing the pressure chamber in accordance with the shrinkage deformation of the piezoelectric layer in the plane direction (unimorph deformation). Accordingly, the volume in the pressure chamber is decreased, and the pressure is applied to the ink contained in the pressure chamber.
The piezoelectric actuator, which has the conventional unimorph structure as described in Japanese Patent Application Laid-open No. 2006-304588, has involved such a problem that the driving efficiency is low. That is, it is necessary that the high driving voltage (difference in electric potential) should be applied between the individual electrode and the vibration plate in order to generate, in the vibration plate, a predetermined amount of the deformation required to apply the desired jetting pressure to the ink. As a result, a problem arises such that the electric power consumption of the actuator is raised or increased.